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This appears to be an attempt at a total power conversion. If you have a mono signal at a given power and played it through a speaker and then compared the volume to two speakers playing the sound, the overall volume would be higher. They are adjusting for that power increase due to two channels being used instead of one, thus the combined signal should have closer to the same power when combined.

This is the more technically correct way to approach the problem if you want to preserve total power which is common on higher end equipment when dealing with this kind of a problem. I'm not sure if other players do the same though most likely some do. The behavior of any particular sound card would depend on how the hardware and drivers for that sound card are implemented and can't be answered generically.